In cellular wireless communication systems, mobile terminals (referred to herein as User Equipment or UE) receive signals transmitted on downlink channels by network transmitters (base stations, or Node B). The UE continuously monitor the channel quality in order to connect to the Node B transmitting the best signal. If the channel quality, as determined by a metric such as the Signal to Interference and Noise Ratio (SINR) goes below a threshold value, the UE typically have problems detecting and reading control and data channels. This in turns means that the UE could miss important information, and/or could misinterpret control signals. For example, a UE erroneously interpreting a control signal may transmit information on a time/frequency/code position reserved for another UE. Thus, UEs out of a coverage area, or cell, of the network could create unnecessary interference. In this case, the UE is said to be out of synchronization with the network.
Prior art Out of Synchronization (OoS) detectors are based on monitoring predetermined control signaling for a particular channel quality metric, such as SINR. Over a predetermined duration T, if the average SINR is below a threshold corresponding to an excessive Block Error Rate (BLER) on control channels (e.g., 10-30% or greater), the UE is deemed to be OoS. The duration T should be chosen such that instantaneous fading dips should not trigger an OoS detection, implying T in the range of 100 ms in currently defined cellular systems.
A formal OoS condition was introduced in Wideband CDMA (WCDMA). The UE monitors the Downlink Dedicated Physical Channel (DL-DPCH) SINR. When the DL-DPCH SINR is too low for reliable control decoding, the UE must cease transmission on the uplink, in order not to interfere with other UEs. Furthermore, due to persistent poor downlink SINR, the UE will transmit power up command resulting in saturation of base station transmit power. One example of such an OoS detection procedure is described in U.S. Pat. No. 7,149,538.
Once a UE is determined to be OoS, it is important that the UE continues to monitor downlink channel quality, so that it may regain synchronization with the network upon receiving a signal of sufficient quality to detect the downlink control signaling. Accurate In Synchronization (IS) detection is important following connection setup or handoff, as well as following OoS detection.
Prior art OoS and IS detection methods do not consider the bandwidth of downlink channels, or transmitter antenna configurations. Advanced wireless communication networks, such as those conforming to the UTRAN LTE (UMTS Terrestrial Radio Access Network Long Term Evolution) protocol, may be configured to operate with a plurality of different defined bandwidths in the downlink. Additionally, LTE supports MIMO (Multiple Input, Multiple Output) techniques employing multiple transmit antennas. Both the bandwidth utilized and the transmit antenna configuration profoundly affect downlink control channel decoding performance.